Arc welding is an efficient manufacturing process for the fabrication of complex structures. However, the utility of such structures is particularly dependent on the quality of the weld. Because of the low weight and close design margins demanded by the aerospace industry, welding quality requirements are particularly stringent in structures used in aircraft engines, aircraft, and spare vehicles. Also, because of high refinement in structural design, it is quite common that irregular and otherwise complex shapes are encountered. These shapes present particular problems since the quality of the weld is generally dependent upon the constancy of welding parameters.
There are many welding processes and the sensitivity of the weld quality to particular parameters varies amongst them. The present invention will be seen to be particularly useful for welding processes which include gas tungsten arc (GTAW), gas metal arc (GMAW) and plasma arc welding (PAW). These processes are particularly suited for welding superalloys where appreciable quantities of filler material must be added. Filler material is needed in welds whenever there is less than the most perfect fitup between the faying surfaces, and to insure a positive weld reinforcement. When filler metal is added, it is usually added by means of feeding a wire into proximity of the arc fusion zone. Under such conditions, the quality of the weld will be dependent on the rate of filler metal addition, the arc voltage and current, and the positioning of the arc welding head over the joint, and other parameters.
As described in the preferred embodiment herein, a diffuser case for a gas turbine as shown in FIG. 1, is a particularly complex structure. Such structures are forged from superalloy and then machined for use; they have various bosses where fittings and the like are attached. These bosses represent thicker areas of the case wall, and rather than making an extra heavy forging which would require extensive machining, the bosses are welded to the wall of the case, as shown in the Figure. It is seen that many of the bosses are irregular in shape and thickness, and lie at different positions along the tapered and curved outer surface of the case. Because of the geometrical problems these bosses have traditionally been welded manually using GTAW and hand-fed filler rod. The process is labor intensive and the welds are prone to having defects whenever there is a small operator deviation. Such defects may be located by radiographic inspection, and they may be repaired by grinding out the weld and rewelding, but such a process is extremely expensive.
Conventional welding machines overcome such operator problems, but they are adapted mostly to welding regular shaped workpieces. For example, Cecil U.S. Pat. No. 4,151,394 illustrates a typical system wherein vertical and horizontal motion of the welding head are obtained. Brown U.S. Pat. No. 4,156,152 discloses plasma arc welding wherein the torch is moved about the x and y axes, in the plane of the workpiece, to follow the weld joint; z axis motion, perpendicular to the plane of the workpiece is used to maintain a desired spacing between the torch and the workpiece. Control of the vertical position is obtained by sensing the arc voltage.
While these motions in the x, y and z directions are known and appear applicable to welding structures such as the aforementioned diffuser case, in fact they are insufficiently sophisticated. This is due to the unusual configuration and requirements for welding of the diffuser case, as are described in more detail below. One aspect includes the need to use GTAW welding to obtain superior quality. As compared to the GMAW process, where the electrode is consumable, in GTAW the filler wire must be separately introduced in front of the weld arc. Thus, when two complete passes around a boss lying on a surface are required, the weld head and connected wire feed unit must rotate at least 720.degree.; with about 810.degree. to include slope out of the weld current. In the GMAW process, such requirements are not encountered, and therefore the prior art equipment is inadequate to the present need. Another aspect in diffuser case welding is the need to closely control parameters, including arc voltage, current, and so forth. As reference to the figures herein will show, the surface of cases is contoured. The wall and boss thicknesses vary, as well. Also, as the case thermally expands during welding, it affects the diameter of the case, and thereby the weld head to workpiece distance. Thus, the critical arc length parameter may vary. Accordingly there is a need for sophisticated control of the welding apparatus motions, to obtain the best weld joint.